In the accompanying manuscript we demonstrate that neutrophils oxidize nearly all of the amino acids commonly found in plasma to a corresponding family of aldehydes in high yield. The reaction is mediated by hypochlorous acid (HOCl), the major oxidant generated by the myeloperoxidase-H2O2-Cl- system of phagocytes. We now present evidence for the underlying mechanism of this reaction, including the structural requirements and reaction intermediates formed. Utilizing mass spectrometry and isotopically labeled amino acids we rule out hydrogen atom abstraction from the (-carbon as the initial event in aldehyde formation during amino acid oxidation, a pathway known to occur with ionizing radiation. Aldehyde generation from amino acids required the presence of an (-amino moiety; (- and (-amino acids did not form aldehydes upon oxidation by either the myeloperoxidase system or HOCl, generating stable monochloramines instead. UV difference spectroscopy, HPLC and multin uclear (1H , 15N) NMR spectroscopy established that the conversion of (-amino acids into aldehydes begins with generation of an unstable (-monochloramine, which subsequently decomposes to yield an aldehyde. Precursor-product relationships between (-amino acid and (-monochloramine, and (-monochloramine and aldehyde were confirmed by HPLC purification of the reaction intermediate and subsequent 1H and 15N NMR spectroscopy. Collectively, these results detail the chemical mechanism and reaction intermediates generated during conversion of amino acids into aldehydes by myeloperoxidase-generated HOCl.